High quality audio signals, such as those generated in the course of many live musical performances or those produced on playback from carefully processed and recorded material, may have a dynamic range of as much as one hundred decibels. A dynamic range of 100 dB constitutes a difference of 100 dB between the quietest sounds discernible and the loudest undistorted sounds represented by the signal. Another instance of a broad dynamic range occurs in the case of sound reinforcement microphones, which are required to handle a dynamic range within the limitations imposed by room background noise and by loud talking in close proximity to the microphones. Here, the dynamic range may exceed 90 dB at the microphone.
Preservation of the full dynamic range for a given audio source, whether in radio broadcasting, in operation of a sound reinforcement system, or in a residential or commercial playback system, is not always desirable and frequently is inordinately expensive. For such situations, compression/limiter amplifiers have been used to develop an audio signal having a restricted dynamic range from an input signal with a substantially broader dynamic range. Thus, broadcast radio stations have used compression/limiter amplifiers to increase the average level of the signal received by listeners. Low level signals in the source material are amplified enough to be heard even if the listener is in a high background noise environment. The compression/limiter amplifier prevents high level signals in the source material from over-modulating the transmitter. Similarly, sound reinforcement systems ideally should compensate for level variations among talkers and for variations in microphone talking distances by providing a more constant and comfortable level in the loudness of the sound reproduced for listeners. A compression/limiter amplifier, adjusted in threshold level, can provide full gain for soft talkers while at the same time reducing the gain for loud talkers.
Another application for compression/limiter amplifiers in audio systems pertains to consumer records and tapes. The dynamic range for a record or a tape is limited by the noise and distortion characterics of the recording medium. A compression/limiter amplifier can be used to restrict the dynamic range prior to recording. In this way, low level sounds are amplified above the recording noise and high amplitude inputs are limited to levels below the recording distortion level.
The reduced dynamic range for the output of a compression/limiter amplifier results in low level signals not falling as low as before the amplifer and high level signals not being as high as before the amplifier. The key to effective design of a useful audio system utilizing a compression/limiter amplifier is to provide a dynamic range restriction having the least audible side effects.
Two different forms of audio systems using compressor/limiter amplifiers have been most commonly employed. In a feed forward compressor/limiter amplifier system, it is customary to employ a voltage-controlled amplifier having a gain characteristic that decreases with increasing DC voltage on its control input. The gain control circuit for an amplifier of this kind converts an incoming audio signal to a DC level proportional to the incoming signal amplitude. This gain control voltage is fed forward to the voltage controlled amplifier so that if the audio signal input decreases in level, the amplifier gain increases. If the audio input signal increases in level, a control signal of greater amplitude is supplied to the voltage-controlled amplifier and the gain is reduced. Consequently, the output level tends to remain constant despite changes in input level. The other commonly employed compression/limiter amplifier circuit is a feedback control. In this type of system, increases in output level from the voltage controlled amplifier increase the feedback control voltage and reduce the amplifier gain, thereby decreasing the output level.
To achieve a flat limiting response in a compression/limiter amplifier of the feedback type, a high gain must be incorporated in the feedback control loop. Consequently, after a transient level change, a feedback limiter amplifier approaches final value in an oscillatory manner that is undesirable for a high quality audio system. A feed forward compression/limiter amplifier, on the other hand, is free of control loop feedback, and hence can provide limiter action without oscillation or "hunting", but a controlled "mild" compression characteristic (e.g. 2:1 compression) is difficult to achieve. Conversely, a feedback compression/limiter amplifier is usually stable for a mild compression characteristic with only limited gain in the feedback loop.
Some attempts have been made to combine the feed forward and feedback controls in a compression/limiter amplifier, but the results have been generally unsatisfactory. These combination circuits have been based upon a summation of the control signal outputs from the feed forward and feedback controls and the result tends to erratic operation of the gain level in the controlled amplifier.